This invention relates to a minute signal amplifier circuit such as a bridge amplifier circuit for amplifying the difference between two input signals, or more in particular to a differential amplifier circuit used for electronic devices for automobiles which is operated by a single power supply of a battery carried on the automobile.
The basic circuit of conventional differential amplifier circuits has the construction as shown in FIG. 1. In FIG. 1, reference numerals 1 and 2 designate input terminals, and an input signal is provided by the difference of potential between the two input terminals. Numeral 3 designates an output terminal, and numeral 4 a grounding terminal. An output signal is provided by the difference in potential between the grounding terminal 4 and the output terminal 3. Numerals 11, 12, 13 and 14 designate resistors, numeral 15 an operational amplifier circuit, numeral 16 a battery, numeral 10 a well-known DC-DC converter. The voltage of the battery 16 is applied through the terminals 5 and 6, so that a positive and negative voltages are produced from the terminal 7 and the terminal 9 respectively, thus actuating the operational amplifier 15. The terminal 8 is a power grounding terminal.
The basic circuit of FIG. 1 is a DC amplifier circuit, and in the case of amplification of a minute signal by several hundred times, the drift of the offset voltage of the operational amplifier 15 is also amplified by several hundred times, resulting in an extremely deteriorated amplification accuracy. A high-class operational amplifier has an offset voltage drift less than 1 .mu.v/.degree. C. but it is high in cost. In general low-cost operational amplifiers, the drift of the offset voltage is 10 to 50 .mu.v/.degree. C.
A circuit system whose differential amplification accuracy is not deteriorated in spite of a low-cost operational amplifier having a large temperature drift of the offset voltage is a chopper system. An example of the chopper system is shown in FIG. 2. An oscillation circuit 110 is for producing a control signal for the chopper and sample holding, and includes a multivibrator (such as CD4047 of RCA) 113, a resistor 112, a capacitor 111 and a NOR gate 114. The multivibrator 113 is connected in such a manner as to act as a non-stable multivibrator, and produces a rectangular pulse signal D of about 8 KHz from the terminal D as shown in FIG. 3(a). On the other hand, a rectangular pulse signal E which is the result of frequency-dividing the pulse signal D by 1/2 as shown in FIG. 3(b) is produced at the terminal E. Further, a pulse signal E0 obtained by reversing the pulse signal E as shown in FIG. 3(c) is produced at the terminal E The NOR gate 114 produces a logic output in response to the pulse signals D and E, and produces a pulse signal F at the output terminal F as shown in FIG. 3(d).
A chopper circuit 60 is comprised of analog switches (such as CD4066 of RCA) 63, 64, 65 and 66 and capacitors 61 and 62. The voltages V.sub.1 and V.sub.2 applied to the input terminals 41 and 42 respectively are converted into rectangular wave signals and produced at the output terminals 45 and 46 respectively. The control terminals of the analog switches 63 and 64 are supplied with the voltage E of the oscillator circuit 110, and the control terminals of the analog switches 65 and 66 with the voltage E0 of the oscillator circuit 110.
Numeral 110 designates an amplifier circuit, and includes two amplifiers 70 and 80. Numeral 70 designates a differential amplifier circuit including resistors 71, 72, 73 and 74 and an operational amplifier circuit 75 for differentially amplifying the output signal of the chopper circuit 60. Numeral 80 designates an amplifier circuit including a capacitor 81, resistors 82, 83 and 84 and an operational amplifier circuit 85 for inverted amplification of the output signal of the amplifier circuit 70.
Numeral 102 shows a power supply for supplying a voltage to the operational amplifiers 75 and 85. Numeral 51 designates a conductor for connecting the positive terminal of the power supply 102 and the operational amplifiers 75 and 85. Numeral 52 shows a power-grounding conductor for connecting the negative terminal of the power supply 102 and the operational amplifiers 75 and 85. Numeral 53 shows a signal grounding conductor for the amplifier circuits 70 and 80. Numeral 101 designates a well-known constant voltage generator or a constant voltage source such as a battery for increasing the potential of the signal grounding conductor 53 by a predetermined voltage as compared with the potential of the power grounding conductor 52.
Numeral 90 designates a sample hold circuit including an analog switch 91 and a hold capacitor 92 for picking up a signal component corresponding to the input signal (the potential difference between the terminals 41 and 42) from the output signal of the amplifier circuit 80. The output signal of the sample hold circuit 90 is produced in the form of a difference between the potentials at the terminals 43 and 44.
The operation of the circuit having the above-mentioned configuration and the operation waveforms of FIG. 3 will be explained. The terminal 41 is supplied with the voltage V.sub.1, and the terminal 42 with the voltage V.sub.2. The analog switches 63 and 64 are turned on at level 1 and turned off at level 0 at the timing shown in FIG. 3(c). The analog switches 65 and 66, on the other hand, are turned on at level 1 and turned off at level 0 at the timing shown in FIG. 3(b). At this timing, the rectangular waves chopped as shown in FIGS. 3(e) and 3(f) are produced at the terminals 45 and 46 respectively. Reference character V.sub.3 designates the voltage of the voltage source 101.
A waveform obtained by differentially amplifying the waveforms of FIGS. 3(e) and 3(f) is produced at the terminal 47. This waveform, as shown in FIG. 3(g), has an average voltage value of V.sub.3 and a crest value of (V.sub.1 -V.sub.2).times.A.sub.1, where A.sub.1 is the amplification factor of the amplifier circuit 70. Though not shown in FIG. 3(g), a DC voltage resulting from multiplying the offset voltage of the operational amplifier 75 by A.sub.1 times is applied to the waveform of FIG. 3(g). In order to block this DC voltage, a capacitor 81 is added to the input circuit of the amplifier circuit 80, thus passing the AC components alone.
A waveform shown in FIG. 3(h) is produced at the terminal 48, which waveform has an average value of V.sub.3 and a crest value of (V.sub.2 -V.sub.1).times.A.sub.1 .times.B.sub.1, where B.sub.1 is the amplification factor of the amplifier circuit 80. The waveform of FIG. 3(h) produced at the terminal 48 is sampled at a predetermined timing by the analog switch 91, held by the capacitor 91, and produced at the terminal 43. As seen from the sampling timing shown in FIG. 3(d), the sampling is taken at level 1. The voltage shown in FIG. 3(i) is produced at the terminal 43. This voltage is higher than the signal grounding voltage V.sub.3 by (V.sub.1 .times.V.sub.2).times.A.sub.1 .times.B.sub.1 /2. Thus an output signal is produced intermediate the terminals 43 and 44.
In the conventional differential amplifier circuits such as the one shown in FIG. 2, the resistors 71, 72, 73 and 74 of highly accurate resistance value and temperature coefficient are required in order to increase the in-phase signal removal ratio; the capacitance values of the capacitors 61 and 62 are required to coincide with each other with high accuracy in order to prevent an overshoot, delayed rise and ringing in the transient characteristics; it is difficult to regulate the in-phase signal removal ratio since equivalent series resistors of the capacitors 61 and 62 are connected in series with the resistors 71 and 72 respectively; a capacitor low in equivalent series resistor, accurate in capacitance and stable in characteristics is required; the in-phase signal removal ratio is deteriorated by the change with time of the resistors 71, 72 and 73 and the capacitors 61 and 62 in an adverse environment of high temperature, high humidity and considerable vibrations in an automobile engine room, thereby leading to the disadvantage of a high cost and low reliability.